In the prior art, and in particular in U.S. Pat. No. 5,562,626, a liquid-injection syringe assembly of the following type is disclosed:                a tubular syringe body forming a reservoir for the liquid, the body being provided with a distal end carrying a liquid-injection needle;        a syringe plunger mounted to be axially movable in the body between a ready position and an end-of-injection position; and        a tubular sheath in which the syringe body is mounted to be axially movable between an active position in which the needle projects from the distal end of the sheath, and a protecting position in which the needle is retracted inside the sheath.        
A syringe assembly of the above type is generally provided with means enabling the needle to be retracted automatically into the sheath so as to ensure that a person handling the syringe assembly after it has been used normally, i.e. after liquid has been injected into the body of a patient, cannot be accidentally pricked by the needle.
When the syringe body is in its protecting position relative to the sheath, it is appropriate to prevent any accidental return of the injection needle towards its position in which it projects from the sheath.
For this purpose, U.S. Pat. No. 5,562,626 purposes axial securing means for securing the syringe body and the sheath axially, said means being activated while the syringe body is in its protecting position, in which the needle is retracted into the sheath. The axial securing means comprise a expansion collar integrally molded with the body of the syringe, which means that said body is specific to the syringe assembly described in U.S. Pat. No. 5,562,626.
Unfortunately, certain medical substances are packaged directly in standard syringe bodies (prefilled syringes), generally made of glass, and closed by pistons which are subsequently connected to plungers for actuating the pistons.